The semiconductor device industry has a market driven need to reduce the time and cost of testing electronic devices such as integrated circuits (ICs), assembled printed circuit boards (PCBs) having various IC and other electronic devices connected together on a ceramic or plastic board, and Micro electro-mechanical (MEM) devices, which may have electronic devices integrated on sensor or actuator substrates. Such testing may include function testing to determine which IC die are properly manufactured, burn in testing to determine if a functional die has what are known as infant mortality problems, hot sort testing to determine the operational speed limits of particular die so that they may be sorted into fast and slow devices, and basic reliability testing where a representative sample of manufactured die are life tested in accelerated environments to determine the expected overall failure rate. In many of these testing cases, the electronic device may have to be heated to a predetermined temperature, such as a normal long term operating temperature, prior to commencement of testing. Thus, one of the factors affecting the amount of time an IC electrical test may consume, and thus the overall cost of testing, is the need to have the IC raised, and perhaps maintained at a predetermined temperature.
It is known to use heated vacuum chucks to hold and heat IC wafers during initial function testing, and to use environmental ovens to control the temperature of packaged IC die and assembled PCBs, but such methods increase the equipment cost, tester space requirements and testing time. What is needed is a method to rapidly and accurately heat an electronic device for electrical testing.
Certain electronic devices may have a normal operating electrical current usage rate, or duty cycle, that changes greatly over relatively short periods of time, with a result that the amount of heating that may occur on the device also changes substantially. Such a device may experience what is known as thermal stress due to the constantly changing device temperature. Thermal stress may cause device package cracking with resultant exposure of the IC to environmental contaminants, and thus an accelerated failure rate. The constant flexing of joints between portions of the electronic device having different coefficients of thermal expansion (CTE) during the thermal changes may result in die to package substrate delamination and thus die failure. The CTE differences between the IC and plastic encapsulant materials during thermal changes may result in ruptured wire bond to IC connections and thus device failure. There are numerous other electronic device failure modes that may be accelerated by thermal changes, resulting in reduced device lifetime. While it is known that higher operating temperatures may cause decreased device reliability and reduced lifetime, a reduction in the amount of thermal cycling a device experiences may result in a greater improvement in the device reliability and lifetime than is lost due to the increase operation temperature. It is known to improve device reliability in high temperature cycle situations by the use of an environmental temperature controller to maintain the electronic device at a more constant temperature, but such devices consume space and electrical power.
As an illustrative example, it is known to test DRAM devices at an elevated temperature to accelerate and expose device failures which may not otherwise have been identified. One known method to preheat a DRAM prior to a test without the need for additional equipment and time, such as a environmental oven, is to use the DRAM circuitry in what may be known as a heat up mode, by repeatedly running the DRAM in auto refresh mode until the desired temperature is obtained. However, DRAMs have been forced in recent years to become lower power users by consumer demand, due to the increased number of DRAMs use in various low power products such as laptop computers, cellular telephones and personal digital assistants, all of which have power limitations since they are battery driven devices. As a result, even if a DRAM is heated up using the auto refresh mode operation prior to a burn in test, the low power consumption of the DRAM may mean that the device temperature may fall during the burn in test, thus invalidating the test.
Thus there exists a need in the art for a simple, fast and accurate method for pre-heating and/or maintaining a desired temperature or temperature profile in an electronic device for both testing and certain operational situations.